1. Field of the Invention
The present invention relates generally to gratings for active fibers, and in particular to a multimode grating for 3-level double-clad fiber and tapered fiber lasers.
2. Technical Background
Optical fiber is the favored transmission medium for telecommunications due to its high capacity and immunity to electrical noise. Silica optical fiber is relatively inexpensive, and when fabricated as a single transverse mode fiber can transmit signals in the 1550 nm band for many kilometers without amplification or regeneration. However, a need still exists for optical amplification in many fiber networks, either because of the great transmission distances involved, or the optical signal being split into many paths.
As illustrated schematically in FIG. 1, a conventional amplifier 10 is interposed between an input transmission fiber 12 and an output transmission fiber 14. Erbium-doped fiber amplifiers (EDFAs) have been found quite effective in providing the required optical gain, as one example of the amplifier 10. Another example of the amplifier 10 is a fiber with Raman gain. Both transmission fibers 12, 14 need to be single-mode, because higher-order modes exhibit much greater dispersion (typically the limiting factor for the fiber transmission distance at high data rates). The EDFA 10 includes a length (on the order of tens of meters) of an erbium-doped silica fiber 16, as is well known in the art. It is well known that an erbium optical fiber amplifier operating in its purely three-level mode is capable, when pumped at a wavelength of 980 nanometers (nm) of amplifying optical signals having a wavelength of 1550 nm. The doped fiber 16 should also be single-mode in order to maintain the transmission signal integrity. The doped fiber 16 is optically active due to the presence of Er3+ ions or other rare-earth ions, which can be excited to higher electronic energy levels when the doped fiber 16 is pumped by a strong optical pump. Typically, an optical pump source 18 inputs the pump into the doped fiber 16 through a pump source fiber 20 coupled to either the undoped upstream fiber 12 or the doped fiber 16 through a wavelength-selective directional coupler 22, but downstream coupling is also known. For efficient coupling into the single-mode Er-doped fiber 16, the pump source fiber 20 should also be single-mode. An operative EDFA may contain some additional elements (such as an isolator or a gain-flattening filter), which are well known to the art but not relevant to the understanding of the background of the present invention.
Conventionally, one typical pump source 18 has been an edge-emitting semiconductor laser that includes a waveguide structure (in what is called a xe2x80x9cstripexe2x80x9d structure) that can be aligned with the single-mode pump source fiber 20 to provide effective power coupling. However, this approach has failed to keep up with modern fiber transmission systems incorporating wavelength-division multiplexing (WDM). In one approach to WDM, a number of independent lasers inject separately modulated optical carrier signals of slightly different wavelengths into the transmission fiber 12. The EDFA has sufficient bandwidth to amplify carrier signals within about a 40 nm bandwidth. A large number of multiplexed signals to be amplified require in aggregate a proportionately large amount of pump power. Over the past decade, the number of WDM channels preferably utilized in a standard network has increased from about four to current levels of forty or more, but at best the output power from a single-stripe laser source has only doubled.
In search for a higher powered laser source, the broad-area diode laser remains the most efficient and least expensive pump source. Recent progress in semiconductor laser technology has led to creation of a broad-area laser diodes with output powers of up to 16 W. Devices 100 xcexcm wide with a slow-axis numerical aperture (NA) of less than 0.1 and output power of 4 Watts at 920 and 980 nm are now passing qualification testing for telecommunication applications. With proper coupling optics, the beam of such a laser diode can be focused into a spot as small as 30xc3x975 xcexcm with an NA of less than 0.35 in both transverse directions. The optical power density in such a spot is xcx9c1.3 MW/cm2, which should be high enough to achieve transparency in 3-level laser systems.
One approach for utilizing inexpensive high-power broad-area pump lasers involves cladding-pumped, or double-clad fiber designs for the optical pump 18. The advantages of cladding-pumped fiber lasers are well known. Such a device effectively serves as a brightness converter, converting a significant part of the multimode pump light into a single-mode output at a longer wavelength.
Cladding pumping can be employed to build a separate high-power single mode fiber pump laser. A source based on the pure three-level 978 nm Yb+3 transition has long been suggested as a pump for EDFAs because this wavelength is close to the desired pumping wavelength of 980 nm. However, the cladding-pumped technique has been determined in practice to be ineffective for pumping pure three-level fiber lasers, such as the 980 nm transition of ytterbium, because of various fiber laser design parameters that have to be satisfied.
Practical double-clad amplifiers and lasers have been mostly limited to 4-level systems. Double-clad fiber lasers offer better performance for four-level lasing (where the lasing occurs in a transition between two excited states) than for the three-level one (where the lasing transition is between the excited and the ground state). For example, for the rare-earth ion, Ytterbium (Yb), the three-level transition is at 978 nm and competing higher-gain four-level transition is at about 1030-1100 nm.
In a double-clad laser, an outer cladding confines the pump light from a primary pump source in a large cross-sectional area multimode inner cladding. The much smaller cross-sectional area core is typically doped with at least one rare-earth ion, for example, neodymium or ytterbium, to provide lasing capability in a single-mode output signal. Typically, a neodymium-doped or ytterbium-doped double-clad fiber is pumped with one or several high-power broad-area diode lasers (at 800 nm or 915 nm) to produce a single transverse mode output (at the neodymium four-level transition of 1060 nm or the ytterbium four level transition of 1030-1120 nm, respectively). Thus, conventional double-clad arrangements facilitate pumping of the fiber using a multimode first cladding for accepting and transferring pump energy to a core along the length of the device. The double-clad laser output can be used to pump a cascaded Raman laser to convert the wavelength to around 1480 nm, which is suitable for pumping erbium. To date, a double-clad design by itself (that is, without an additional Raman converter) does not produce a sufficiently high output in any of the appropriate absorption bands for EDFAs or is not available commercially.
How much pump light can be coupled into a double-clad fiber inner cladding depends on the cladding size and NA. As is known, the xe2x80x9cetenduexe2x80x9d (numerical aperture multiplied by the aperture dimension or spot size) of the fiber should be equal to or greater than the etendue of the pump source for efficient coupling. The numerical aperture and spot size may be different in both axes so there may be an etendue in the x and y directions that must be maintained or exceeded.
Typically, a high numerical aperture NAclad, related to the difference in refractive index between the first and second cladding is desired. If there are two claddings instead of one, the index of the first cladding is nclad,1 and the index of the second cladding is nclad,2 such that NAclad=(nclad,12xe2x88x92nclad,22)1/2. In the well-known design, the first clad layer is made of glass and the second is made of plastic (fluorinated polymer) with a relatively low refractive index in order to increase the numerical aperture NAclad. Such plastic may not have the desired thermal stability for many applications, may delaminate from the first cladding, and may be susceptible to moisture damage.
In known double-clad host fibers, the laser cavity is formed by an input dielectric mirror which transmits the 920-nm pump band and reflects the desired 980-nm lasing band. For any input mirror of the fiber laser, it is a desire to reflect only the fundamental mode, at the laser wavelength, e.g., 978 nm, to form the input end of the optical cavity. A dielectric mirror at the end of the double-clad fiber or a weak fiber Bragg grating in the single-mode fiber, e.g., Corning(copyright) CS-980 fiber, coupled to the coupling end of the double-clad fiber serves as the output coupler for providing the output end of the cavity.
One of the primary technical challenges in a high power fiber laser is the formation of the input dielectric mirror across the multimode inner cladding of the double-clad fiber. Approaches include attaching a glass micro-sheet to the fiber endface or directly depositing a thin-film dielectric on the fiber endface, but both of these methods present their own technical hurdles.
A two-stage fiber laser has also been proposed as an alternate optical pump 18. This two-stage laser has an optical pump source to provide a pump light at a pump wavelength. A first waveguide portion which when optically pumped at the pump wavelength is capable of lasing with an emission at a lasing wavelength. The first waveguide portion exhibits multi-transverse-mode behavior at the lasing wavelength. A second waveguide portion exhibiting a substantially single transverse mode behavior at the lasing wavelength is optically coupled together with the first waveguide portion. An optical cavity is defined by a multimode grating on the first waveguide portion and a single mode grating on the second waveguide portion and includes the first and second waveguide portions. The delta index or contrast index of the difference between the cladding refractive index and the multimode core refractive index is between 0.04 to 0.06 for the low indexed germania (Ge) doped silicates multimode fibers of this approach.
As is known, the terminology xe2x80x9cfiber Bragg gratingxe2x80x9d refers to a grating in which incident light is reflected back along the same fiber by a xe2x80x9cshort periodxe2x80x9d (a.k.a. Bragg) grating in the fiber and the fabrication of gratings is known. Fiber Bragg gratings (FBGs) couple power from one mode to another provided that the propagation constants of the two modes satisfy the following grating equation:                                           β            1                    -                      β            2                          =                              2            ⁢            π                    Λ                                    Eq        .                  xe2x80x83                ⁢                  (          1          )                    
where xcex21 and xcex22 are the propagation constants of the two modes, A is the grating period in the fiber, and first order diffraction is assumed for simplicity. When a forward propagating mode reflects into the identical backwards propagating mode, the Bragg condition becomes xcexB=2neffxcex9, where neff is the effective index of the mode (xcex2=(2xcfx80/xcex)neff) and lies between the core index ncore and the cladding index, nclad for guided modes (nclad less than neff less than ncore). Forward propagating modes may also reflect into other modes when mode orthogonality is no longer maintained, for example when UV induced index changes due to the FBG itself perturb the index profile sufficiently. The index profile needed depends on fiber geometry, cladding material, and the exact wavelengths for the particular application.
As with the double-clad fiber laser, to enable the maximum launch of optical power from the high power pump source into the laser cavity of either the double-clad fiber or the two-stage multimode to single-mode fiber laser, the optical cavity needs to have a large numerical aperture (NA) which is related to the index contrast. However, an increased index delta for proving power enhancement requires more design, testing, and manufacturing complexities to be first solved.
Therefore there is a continued need to increase the power output of a fiber laser, whether double-clad or two-staged, while increasing the reliability and simplifying the packaging and manufacturing of the fiber laser, which will also reduce the cost of the fiber laser.
In an embodiment of the invention, an optically-active waveguide laser includes a multimode portion for carrying more than one spatial mode at a predetermined wavelength. The multimode portion has a first refractive index. A cladding portion proximate the multimode portion has a second refractive index lower than the first index by at least 0.1 for power enhancement. A multimode grating is written on at least one section of the multimode portion for reflecting the predetermined wavelength.
In another aspect of the invention, the optically active fiber is used for making a fiber laser. This double-clad structured active fiber has a core doped with an optically excitable ion having a three-level transition. The core has a core refractive index and a core cross-sectional area. An inner cladding surrounds the core. The inner cladding has an inner cladding refractive index less than the core refractive index, an inner cladding cross-sectional area between 2 and 25 times greater than that of the core cross-sectional area, and an aspect ratio greater than 1.5:1. An outer cladding surrounds the inner cladding and has an outer cladding refractive index less than the inner cladding refractive index.
The fiber laser or amplifier includes an optical fiber having a core doped with an ion such as erbium (Er3+), neodymium (Nd3+) or ytterbium (Yb3+), which can be optically pumped.
In another aspect of the invention, the active fiber and a single-mode output fiber are both designed to provide equal mode field diameters for the lowest-order mode at the junction and the multimoded photosensitivity-doped inner cladding cross-section has an elongated shape with an aspect ratio matching that of the pumping diode laser where at least one multimode grating is written.
The invention is particularly advantageous when used as a pump source for an erbium-doped fiber amplifier (EDFA) or for the EDFA itself, such as may be found in single-mode fiber optic communication systems or networks.